The long-term goal of this research is to understand the visual processing in the inner plexiform layer of the retina. More immediately, the research serves to provide genetic access to distinct amacrine cell types for functional characterization and to understand how they shape the response properties of ganglion cells. Amacrine cells are the most diverse neurons in the retina, at least 40?50 morphological types have been described. Each type of amacrine cells exhibits a unique morphology and generates specific visual computations through their local circuits. Unfortunately, the great diversity of amacrine cells has been a major obstacle to access individual cell types for systematic studies. As a result, the connectivity and function of many amacrine cells remain unknown, and the development of genetic tools that allow for cell type-specific targeting and manipulation would be an important step towards their characterization. We propose to use new mouse intersectional genetic tools combined with functional imaging and electrophysiology recording to morphologically and functionally dissect amacrine cell circuits in three separate Aims. In Aim 1, we will create intersectional strategies by using a combination of Cre and tTA expression to discover new amacrine cell types and to target these cells with increased specificity. After that, we will focus on a newly discovered amacrine cell type called Ck2-AC1 for functional analysis. We will characterize the light responses of Ck2-AC1s by imaging Ca2+ responses at the sites of neurotransmitter release in Aim 1, and then identify their post-synaptic ganglion cells with intersectional ChR2 activation in Aim 2. In Aim 3, we will test specific hypotheses about Ck2-AC1s and examine their functional roles in different circuits with chemogenetic inactivation. We will use both hypothesis driven and discovery based approaches to gain insights into the circuit functions of Ck2-AC1s in the inner retina. The intersectional strategy is extensible, and we will undoubted discover additional novel amacrine cells and circuits utilizing the methods established for Ck2-AC1s. This work will advance our understanding of visual processing in the inner retina while the technologies developed will provide major advances over existing methods for studying amacrine cells and are also applicable to other brain circuits. Furthermore, this project will provide insight into pathophysiological neuronal mechanisms of retinal diseases, and help design better strategies for therapy.